1. Field of the Invention
The invention relates to a technical field of a secondary vibration damping type crystal oscillator circuit which is formed as a Colpitts type oscillator circuit, and in particular, to a crystal oscillator circuit whose crystal unit is formed as an SC-cut crystal element to damp oscillation at a vibrational frequency in B mode.
2. Description of the Related Art
Crystal oscillator circuits have extremely high frequency stability due to their crystal units, to be adopted as sources of generating frequencies for various communication devices. As one of these, there is, for example, a crystal oscillator circuit using an SC-cut crystal unit excellent in stress sensitivity behavior, and this is particularly applied to, for example, a base station or the like whose frequency deviation is set in 1 ppb (parts per billion unit) to be highly-stabilized.
FIGS. 5A and 5B are diagrams of for explanation of one example of related art crystal oscillator circuits. FIGS. 5A and 5B are both high-frequency type crystal oscillator circuit (equivalent circuit) diagrams from which their power supplies, bias resistances, and the like are eliminated.
The crystal oscillator circuit of FIG. 5A has an oscillating resonant circuit 1 and an oscillating amplifier 2 that feedback-amplifies an oscillating frequency dependent on the oscillating resonant circuit 1. The oscillating resonant circuit 1 is a Colpitts type oscillator circuit in general, and is basically composed of a crystal unit 3 functioning as an inductor component and first and second capacitors C1 and C2 serving as dividing (split) capacitors. The oscillating amplifier 2 is composed of, for example, a transistor 2A, and the crystal unit 3 is connected between base and collector terminals, the first capacitor C1 is connected between emitter and collector terminals, and the second capacitor C2 is connected between emitter and base terminals.
For example, with a base potential of the oscillating transistor 2A as a standard, the terminals of the oscillating resonant circuit 1 are connected between the base and collector terminals, and a midpoint (series-connected point) of the first and second capacitors C1 and C2 is connected to the emitter. Then, a voltage between the terminals of the oscillating resonant circuit 1 is set to an oscillation output Vout, and a part of the oscillation output Vout divided by the first and second capacitors C1 and C2 is fed back to the region between the base and emitter terminals to be an input Vi. Thereby, the oscillating transistor 2A amplifies a part (input Vi) of the oscillation output, and these operations are repeated to continue oscillation.
Normally, because its output impedance can be decreased to make coupling with the subsequent stage easy, a common collector is adopted (FIG. 6A). Reference numerals and letters R1 and R2 in the drawing denote base and bias resistances, reference numeral and letter R3 is a load resistance, and reference letter Vcc is a power supply. Note that even in this case, it is a matter of course that the circuit diagram is the same as the oscillator circuit diagram of FIG. 5A described above at a high frequency.
The crystal unit (crystal element) 3 is formed as an SC-cut crystal element which is typical as a so-called double rotation Y cut system in which the Y-axis is rotated twice around the X-axis and the Z-axis of the crystal axes (XYZ). In the SC-cut crystal unit 3, as shown in a reactance characteristics diagram of FIG. 7, C mode as thickness-shear vibration is principal vibration, and normally, secondary vibration in B mode whose displacement direction is perpendicular to the principal vibration is generated. As is clear from an equivalent circuit of crystal unit (not shown), these principal vibration (C mode) and secondary vibration (B mode) both have resonance points (resonant frequencies) f1 and f2 and antiresonance points (antiresonant frequencies) f1′ and f2′.
In this case, the C mode and the B mode come close to each other, and the B mode comes to a higher frequency side by approximately 10% of the C mode, and its resonant level at the resonance point f2 is made the same or greater than that at the resonant point f1, which means that its crystal impedance (CI) is made the same or less than that in C mode. Then, in any case, the regions between the resonance points f1 and f2 and the antiresonance points f1′ and f2′ are considered as inductor regions ΔF1 and ΔF2, and the inductor region ΔF1 in C mode and the first and second capacitors C1 and C2 form the oscillating resonant circuit 1.
Thereby, a resonant frequency is determined on the basis of inductance at the vibrational frequency f1o in the inductor region ΔF1 and a composite capacitance of the first and second capacitors C1 and C2, and the vibrational frequency f1o in the inductor region ΔF1 is an oscillating frequency. However, an oscillating frequency is determined on the basis of the inductance, the composite capacitance, and a serial equivalent capacitance in which all the capacitances at the circuit side including the oscillating transistor 2A when viewed from the crystal unit 3 are added, and the oscillating frequency approximately corresponds to an oscillating frequency fo.
Accordingly, the inductor region ΔF1 (f1-f1′) between the resonance point f1 and the antiresonance point f′ in principal vibration (C mode) is a basic oscillation region in C mode. Then, this is the same as in secondary vibration (in B mode) as well, and the inductor region ΔF2 in B mode is an oscillation region in which oscillation is possible. Note that, in practice, the oscillation regions in the respective modes are narrower than the inductor regions ΔF1 and ΔF2.
On the other hand, as described above, the B mode comes close to the C mode, and its resonant level is made the same or greater than that in C mode, which means that its CI is made the same or less than that in C mode. Accordingly, it is difficult to set a circuit constant or the like in a case of oscillation at only the vibrational frequency f1o in the inductor region ΔF1 in C mode that is principal vibration, which brings about oscillation in B mode (at the vibrational frequency f2o in the inductor region ΔF2).
For this reason, for example, in JP-A-2006-345115, as shown in FIG. 6B, a first LC series circuit 4x composed of an inductor L and a capacitor C is connected between the midpoint of the first and second capacitors C1 and C2 and the emitter, and a second LC series circuit 4y which is the same as the first LC series circuit 4x is connected between the emitter and the ground (collector). Incidentally, even in this case, the circuit diagram is the same as the oscillator circuit diagram of FIG. 5B described above at a high frequency. A voltage applied to the first LC series circuit 4x is approximately corresponded to the vibrational frequency (i.e., oscillating frequency) f1o in the oscillation region ΔF1 in C mode, and a voltage applied to the second LC series circuit 4y is approximately corresponded to the vibrational frequency f2o in the oscillation region ΔF2 in B mode.
Thereby, the first LC series circuit 4x feeds back the oscillating frequency (vibrational frequency) f1o in C mode and frequency components in the vicinity thereof to the base via the midpoint of the first and second capacitors C1 and C2 from the emitter. Then, the first LC series circuit 4x suppresses the frequency domain (components) other than those, to form a sort of passband for C mode. Further, the second LC series circuit 4y forms a region for damping B mode so as to bring frequency components in the oscillation region ΔF2 in B mode from the emitter down to the ground potential to eliminate those from the feedback loop to the base. Accordingly, it is possible to damp oscillation in B mode, which makes certain of oscillation at the vibrational frequency f1o in C mode.
In this case, with respect to the negative resistance characteristics at the circuit side when viewed from the crystal unit 3, as shown in FIG. 8, given that a resistance value is 0 at a frequency fα between the oscillation region ΔF1 in principal vibration (C mode) and the oscillation region ΔF2 in secondary vibration (B mode), the characteristics become negative resistance in the oscillation region ΔF1 in C mode, and become positive resistance in the oscillation region ΔF2 in B mode. Accordingly, the oscillator circuit satisfies its electricity condition that is one of the oscillation conditions, to generate oscillation in C mode. However, the oscillator circuit does not satisfy the electricity condition in B mode, and does not generate oscillation.
Incidentally, JP-A-2000-295037 and JP-A-9-153740 also disclose related art crystal oscillator circuits.
However, in the related crystal oscillator circuits (B mode damping) having the above-described configurations, because the first and second series resonant circuits 4 (x and y) forming a passband for C mode that is principal vibration and a region for damping B mode that is secondary vibration are inserted into the feedback loop, there is a problem that the number of components is increased. Further, in the case of JP-A-2000-295037 as well, a passband for C mode and a region for damping B mode are formed for the same purpose. However, in this case as well, there is the problem that a number of components is increased in the same way.